1. Field of the Invention
The present invention relates to an atomized thin film forming apparatus for forming a thin film over the surface of a substrate by spraying an atomized source solution over the surface of the heated substrate and, more specifically, to an atomized thin film forming apparatus capable of forming a conductive film over the surface of the heated substrate and preventing the thickness of variation in the conductive film from being large.
2. Prior Art
A transparent conductive film to be employed in a solar cell, liquid crystal display, a plasma display panel, and the like, is formed of a thin film composed of Tin oxide (SnO.sub.2) or Indium Tin Oxide (ITO). The transparent conductive film is formed by spouting a mist of an atomized source solution generated by an atomizer toward heated substrates through a film forming nozzle and reacting the atomized source solution on the surface of the heated substrate. An example of a prior art atomized thin film forming apparatus for forming a thin film employed for forming the transparent conductive film according to the method mentioned just above will be described hereinafter. The source solution is atomized by an atomizer and the resultant atomized source solution is spouted from a opening of a film forming nozzle. There is provided a film forming chamber over the spouting opening of the film forming nozzle. The mist of the atomized source solution floats in the film forming chamber. The substrates are delivered successively and continuously into the film forming chamber so that the surfaces of the substrates form a ceiling of the film forming chamber. The substrates are heated to a predetermined temperature by a heater disposed at the rear side of the substrates via a uniform heating plate.
In conventional apparatuses, the substrates such as glass and the like are introduced successively from a substrate inlet of the film forming chamber and delivered into the film forming chamber and discharged from the substrate outlet of the film forming chamber. In the film forming chamber, the film forming nozzle is disposed close to the main surfaces of the substrates at the tip end thereof. The mist of the atomized source solution spouted from the film forming nozzle flows gently toward an exhaust duct of the film forming chamber. While floating toward the exhaust duct, material in the source solution reacts with the oxygen in the air or moisture in the source solution, whereby an oxide accumulates on the surfaces of the substrates to form an oxide film thereon. The mist of the source solution which did not contribute to the formation of the thin film is discharged from the exhaust duct.
The condition of the mist of the source solution when spouted from the film forming nozzle to the film forming chamber is very important factor in effectively forming a film of high uniformity. More in detail, in order to form a film of high uniformity over the surfaces of the substrates, the flow of the mist of the source solution must to be stratified in the film forming chamber. To meet this requirement, it is preferable that the width of the spouting opening of the film forming nozzle from which the mist of the source solution is spouted is, to some extent narrow. However, if the width of the film forming nozzle is restricted as a whole, the resistance of the inner wall surface of the film forming nozzle relative to the flow of the mist of the source solution is increased, thereby reducing the amount of mist of the atomized source solution to be stably supplied in the stratified state per hour. Hence, film forming speed is reduced so that the high productivity can not be obtained. Accordingly, there is proposed a film forming nozzle having an upper end diameter of which is restricted at the appropriate inclination.
However, when the transparent conductive film is formed over the surface of substrates such as glass by conventional film forming apparatus, the uniformity of the thickness of the conductive film is of high just after the start of the formation of the conductive film but as the formation of the film continues, i.e., after the lapse of time, the uniformity of the thickness of the conductive film varies. There appears an interference streak on the conductive thin film having such a variety of thicknesses, it is not necessary properties to obtain the property necessary for the transparent conductive film. This streak is caused be the to restriction of the upper end of the film forming nozzle. That is, due to the restriction of the upper end of the film forming nozzle, there is defined locally a corner portion of the inner wall surface of the film forming nozzle which has an angle which varies precipitously and not gently and the mist of the source solution stagnates on the precipitous corner portion so that the mist of the source generates a precipitate which thereafter grows. Due to the growth of the precipitate, the source solution is prevented from flowing locally.
To overcome the problem set forth above, it is necessary to clean the inner wall of the film forming nozzle or exchange the film forming nozzle with another nozzle before the precipitate of the atomized source solution affects the uniformity of the thickness of the thin film. However, there was a problem in that it was necessary to stop the film forming apparatus each time when the film forming nozzle is cleaned or the film forming nozzle is exchanged with another nozzle, which reduces the working rate of the apparatus and prevents high productivity.